The present invention relates to a process for preparing a composite ion exchange membrane containing both anion and cation exchange groups. More specifically, the present invention relates to a process for preparing a composite amphoteric ion exchange membrane which can be obtained by forming a thin layer less than several microns containing both anion and cation exchange groups on a microporous polymer substrate having a selective permeability of its own.
It has been known that the membrane which has anion and cation exchange groups separately without forming any salts provides characteristic effects in permeability. For example, piezodialysis is a typical example of this type of effect. According to the research of the present inventors, it has become clear that a membrane having properly combined anion and cation exchange groups also shows selective permeability for inorganic ions. The membrane having such an amphoteric property, therefore, will be able to separate a low molecular organic substance from an inorganic salt, acid, or alkali as well as salt from acid, or salt from alkali.
By the method already known in U.S. Pat. No. 3,945,927, composite ion exchange membranes having anion and cation exchange groups can be obtained resulting in considerably higher mechanical strength. However, because an anion, or a cation group is introduced into a non-polar polymer membrane, the process and conditions of the reaction naturally become complex and various difficulties are apt to follow.
The present inventors have made an extensive study to eliminate these drawbacks and finally completed the present invention, which comprises forming a thin polymer membrane having either an anion or a cation exchange group onto a microporous polymer substrate and introducing another ion exchange group of an opposite sign into the polymer membrane. Thus, the present invention has made it possible to simplify the process and conditions of the reaction of a large extent.